Atmos. Chem. Phys., 15, 12065–12078, 2015

www.atmos-chem-phys.net/15/12065/2015/

doi:10.5194/acp-15-12065-2015

© Author(s) 2015. CC Attribution 3.0 License.

The regional distribution characteristics of aerosol optical depth

over the Tibetan Plateau

C. Xu1,2,3,a, Y. M. Ma1,2,3,a, C. You1,2, and Z. K. Zhu1,2,3

1Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research,

CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing 100101, China2University of Chinese Academy of Sciences, Beijing 100049, China3Qomolangma Station for Atmospheric Environmental Observation and Research, Chinese Academy of Sciences,

Dingri 858200, Tibet, Chinaanow at: Institute of Tibetan Plateau Research, CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese

Academy of Sciences, Beijing, China

Correspondence to: Y. M. Ma (ymma@itpcas.ac.cn) and C. Xu (xuchao@itpcas.ac.cn)

Received: 4 May 2015 – Published in Atmos. Chem. Phys. Discuss.: 11 June 2015

Revised: 17 October 2015 – Accepted: 23 October 2015 – Published: 30 October 2015

Abstract. The Tibetan Plateau (TP) is representative of typ-

ical clean atmospheric conditions. Aerosol optical depth

(AOD) retrieved by the Multi-angle Imaging SpectroRa-

diometer (MISR) is higher over Qaidam Basin than the rest

of the TP throughout the year. Different monthly variation

patterns of AOD are observed over the southern and north-

ern TP, whereby the aerosol load is usually higher in the

northern TP than in the southern part. The aerosol load over

the northern part increases from April to June, peaking in

May. The maximum concentration of aerosols over the south-

ern TP occurs in July. Aerosols appear to be more easily

transported to the main body of the TP across the northern

edge rather than the southern edge. This is maybe partly be-

cause the altitude is lower at the northern edge than that of

the Himalayas located along the southern edge of the TP.

Three-dimensional distributions of dust, polluted dust, pol-

luted continental aerosol and smoke are also investigated,

based on Cloud-Aerosol Lidar and Infrared Pathfinder Satel-

lite Observation (CALIPSO) data. Dust is found to be the

most prominent aerosol type on the TP, and other types of

aerosols affect the atmospheric environment slightly. A di-

viding line of higher dust occurrence in the northern TP and

lower dust occurrence in the southern TP can be observed

clearly at an altitude of 6–8 km above sea level, especially

in spring and summer. This demarcation appears around 33–

35◦ N in the middle of the plateau, and it is possibly associ-

ated with the high-altitude terrain in the same geographic lo-

cation. Comparisons of CALIPSO and MISR data show that

the vertical dust occurrences are consistent with the spatial

patterns of AOD. The different seasonal variation patterns

between the northern and southern TP are primarily driven

by atmospheric circulation, and are also related to the emis-

sion characteristics over the surrounding regions.

1 Introduction

The Tibetan Plateau (TP), located in central eastern Eura-

sia, is the most prominent and complex terrain feature on

the Earth. It has the world’s highest average elevation (about

4000 m), with some surface features even reaching into the

mid-troposphere (Fig. 1). The TP is surrounded by several

deserts, including the Taklamakan Desert in the Tarim Basin,

Gobi Desert and the deserts in southwest Asia and the Mid-

dle East. The Indo-Gangetic Plain is located to the south

of the TP, with high aerosol loading (Gautam et al., 2011).

Several mountains are located on the TP, including the Hi-

malayas, Gangdise, Nyainqêntanglha, Tanggula and Kunlun

mountains. The elevation differences of these mountains are

at least 500 m and usually 1000 m or even more compared

with the surrounding areas. Due to its topographic charac-

teristics, the TP surface absorbs high quantities of solar ra-

diation with corresponding impacts on surface heat or water

fluxes (Ma et al., 2014a, b). The east Asian monsoon and the

Published by Copernicus Publications on behalf of the European Geosciences Union.

12066 C. Xu et al.: The regional distribution characteristics of AOD

Figure 1. The topography (in meters) of the Tibetan Plateau and

main mountain ranges on the Tibetan Plateau.

eastern part of the south Asian monsoon systems are mainly

controlled by the thermal forcing of the TP (Wu et al., 2007,

2012).

Many studies have focused on environmental and climate

change over the TP (Xu et al., 2009; Ma et al., 2011; Lin

et al., 2012; Yao et al., 2012; Sheng et al., 2013), and the

TP environment is greatly affected by natural and anthro-

pogenic aerosols from the surrounding regions (Z. Liu et

al., 2008; Bucci et al., 2014; Cong et al., 2015). Therefore,

studying tropospheric aerosols and their effects on the TP

is of great importance (King et al., 1999; Kaufman et al.,

2002; Li et al., 2011). Vernier et al. (2011) reported the

presence of an aerosol layer at the tropopause level above

Asia during the monsoon season. The Taklamakan and Gobi

deserts are two major dust sources with long-range transport

mainly occurring in spring (D. Liu et al., 2008). Summer-

time Tibetan airborne dust plumes were detected from the

Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Obser-

vations (CALIPSO) satellite (Huang et al., 2007), and Xia

et al. (2008) suggested that the aerosol load in summer over

the TP was mainly associated with the Taklamakan Desert.

Dust above the TP appears to be largely related to source re-

gions to the north and on the eastern part of the TP (Z. Liu

et al., 2008). The impact of aerosols above and around the

TP during pre-monsoon season was also investigated; how-

ever, a strong elevated heating which could influence large-

scale monsoonal circulations was not found (Kuhlmann and

Quaas, 2010). Atmospheric brown clouds over south Asia,

resulting from biomass burning and fossil fuel consump-

tion, are recognized as a serious environmental problem (Ra-

manathan et al., 2005). These carbonaceous aerosols lead to

a large reduction of solar radiation at the surface, an increase

of solar heating in the atmosphere and a weaker hydrolog-

ical cycle (Ramanathan et al., 2001). Anthropogenic emis-

sions from strong pollution events can occasionally be trans-

ported to the central TP by prevailing southwesterly winds

(Xia et al., 2011). The high altitudes of the Himalayas ap-

pear to block most BC particles intruding into the TP, but the

Yarlung Tsangpo River valley serves as a “leak” by which

contaminants can reach the southeast TP (Cao et al., 2010).

Results from precipitation isotope observations revealed that

the northward maximum extent of the southwest monsoon

over the Tibetan Plateau is located around 34–35◦ N (Tian

et al., 2007). The southern and northern TP are under the

control of different climate systems during the monsoon sea-

son (Yao et al., 2013). Some previous studies have indicated

that the southern and northern TP possibly present different

variations in aerosol properties, e.g., different temporal vari-

ations in dust storm records (Wu et al., 2013). However, the

mechanisms of those differences still need further study.

Although the aerosol load is relatively low, aerosols over

the TP have unique characteristics. In this study, the seasonal

variations and spatial distributions of aerosols over the TP

are presented based on the Multi-angle Imaging SpectroRa-

diometer (MISR) data. The seasonal vertical distributions of

dust, polluted dust, polluted continental aerosol and smoke

are also investigated using CALIPSO data. This study might

indicate that a natural demarcation of aerosols between the

northern and southern TP exists in the middle of the plateau.

In addition to that, the spatial patterns of aerosol loading are

consistent with the vertical distributions of aerosols. We pre-

liminarily propose the possible mechanisms for the aerosol

distributions.

2 Data and methodology

The MISR was successfully launched into Sun-synchronous

polar orbit aboard Terra, NASA’s first Earth Observing Sys-

tem (EOS) spacecraft, on 18 December 1999. Viewing the

sunlit Earth simultaneously at nine widely spaced angles,

MISR provides radiometrically and geometrically calibrated

images in four spectral bands at each of the angles. MISR ob-

serves the entire Earth about once per week. The spatial res-

olution of the operational MISR aerosol retrieval algorithm

is 17.6 km× 17.6 km. The retrieval region has 16× 16 sub-

regions, and each subregion covers a 1.1 km× 1.1 km area.

One of the key issues for satellite aerosol products is cloud

contamination, including MISR data (Kahn et al., 2010).

Three cloud-mask products are used in the aerosol prepro-

cessing. The MISR standard products include three sepa-

rate MISR-derived cloud masks: Radiometric Camera-by-

camera Cloud Mask (RCCM) (Yang et al., 2007), Stereo-

Derived Cloud Mask (SDCM) (Moroney et al., 2002) and

Angular-Signature Cloud Mask (ASCM) (Di Girolamo and

Wilson, 2003). Based on collocated MISR and Moderate

Resolution Imaging Spectroradiometer (MODIS) data, Shi

et al. (2014) suggested that cloud contamination existed in

both over-water and over-land MISR AOD data, with heavier

cloud contamination occurring over the high-latitude south-

Atmos. Chem. Phys., 15, 12065–12078, 2015 www.atmos-chem-phys.net/15/12065/2015/

C. Xu et al.: The regional distribution characteristics of AOD 12067

ern hemispheric oceans. MISR aerosol retrievals have been

evaluated by many studies (Martonchik et al., 2004; Kahn,

2005; Witek et al., 2013). The accuracy of MISR AOD was

much better than MODIS AOD over land (Abdou et al.,

2005). Xia et al. (2008) made comparisons of MISR AOD

with ground-based hazemeter measurements made at Lhasa

and Haibei stations on the TP, which showed a high cor-

relation coefficient and a low root-mean-square error. The

Level 3 aerosol product is a summary of the Level 2 aerosol

product. In this study, daily Level 3 MISR aerosol data

from March 2000 to December 2014 are used to investigate

the aerosol spatial distribution, and the spatial resolution of

MISR data is 0.5◦× 0.5◦.

Monthly variations of AOD at 558 nm are analyzed over

the TP. If there was only one satellite observation for a month

then this grid value in that particular month was excluded for

that year, since there would be too few observations to repre-

sent the monthly average. The monthly means that represent

the aerosol long-term distribution are calculated by averag-

ing the observation data for that month in each year. To an-

alyze aerosol zonal average, only aerosol retrievals over the

TP (27–40◦ N, 75–105◦ E) at elevations higher than 3000 m

are used. In this study, the northern part of the TP is defined

as the region north of 33–34◦ N, and the southern part of the

TP is defined as the region south of 33–34◦ N. The reason for

selecting 33–34◦ N is based on the distributions of aerosols,

which appear to show different patterns to the north and south

of this latitude.

The Cloud-Aerosol Lidar and Infrared Pathfinder Satel-

lite Observation (CALIPSO) satellite provides new insight

into clouds and atmospheric aerosols (Winker et al., 2007,

2010). The CALIPSO satellite was launched into a Sun-

synchronous orbit on 28 April 2006, with a 16-day repeating

cycle. The lidar Level 3 aerosol product is a quality-screened

aggregation of Level 2 aerosol profile data. A series of fil-

ters are designed to eliminate samples and layers that were

detected or classified with very low confidence or that have

untrustworthy extinction retrievals (Winker et al., 2013). The

CALIPSO version 1.00 and version 1.30 Level 3 aerosol pro-

file data from March 2007 to February 2015 are used in this

study. Nighttime all-sky data are used, because the instru-

ment is more sensitive during nighttime than daytime without

solar background illumination (Winker et al., 2013). The spa-

tial resolution of Level 3 data is 5◦× 2◦ (longitude–latitude),

and the vertical resolution is 60 m observed from −0.5 to

12 km above sea level (a.s.l.; all altitudes hereafter also re-

fer to a.s.l. where no other specification is given) in the tro-

posphere. The primary variable of aerosol type is used, and

this variable counts the number of aerosol samples of each

aerosol type for each latitude/longitude/altitude grid cell. Six

aerosol types including clean marine, dust, polluted conti-

nental aerosol, clean continental aerosol, polluted dust, and

smoke are classified. From the data product descriptions, the

composition of six aerosol types can be known. Clean ma-

rine is a hygroscopic aerosol that consists primarily of sea

salt (NaCl). Dust is mostly mineral soil. Polluted continental

aerosol is background aerosol with a substantial fraction of

urban pollution. Clean continental aerosol is a lightly loaded

aerosol consisting of sulfates (SO2−4 ), nitrates (NO−3 ), or-

ganic carbon (OC) or ammonium (NH+4 ). Polluted dust is a

mixture of desert dust and smoke or urban pollution. Smoke

aerosol consists primarily of soot and OC. Mixtures of two

aerosol types are not assigned to one aerosol sample. Dust,

polluted dust, polluted continental aerosol and smoke pos-

sibly influence the TP (Kuhlmann and Quaas, 2010). The

aerosol samples for these major types in each season were

calculated by accumulating the detected samples for the sea-

son in each latitude/longitude/altitude grid cell. The multi-

year aerosol samples for each season are obtained by av-

eraging across multiple years. The classification algorithms

use the integrated attenuated backscatter measurements, the

volume depolarization ratio measurements, surface type and

layer altitude to determine aerosol type (Omar et al., 2009).

The similarity of the optical properties between polluted con-

tinental aerosol and smoke makes the classification of these

two aerosol types difficult (Omar et al., 2009). Mielonen et

al. (2009) made comparisons of Cloud-Aerosol Lidar with

Orthogonal Polarization (CALIOP) Level 2 aerosol types and

those derived from Aerosol Robotic Network (AERONET)

inversion data. The results revealed the greatest agreement

for the dust type (91 % of the cases), moderate agreement

for the polluted dust type (53 % of the cases), and poorer

agreement for smoke (37 % of the cases) and for polluted

and clean continental aerosol combined (22 % of the cases).

Burton et al. (2013) made comparisons of aerosol types be-

tween CALIPSO and airborne High Spectral Resolution Li-

dar, which showed the best agreement for desert dust (80 %

of the cases) and marine aerosols (62 % of the cases), mod-

erate agreement for the polluted continental aerosols (53 %

of the cases), but relatively poor agreement for polluted dust

(35 % of the cases) and smoke (13 % of the cases). Although

previous studies showed different results of quantitative val-

idations, these research studies indicated that the classifica-

tions for dust aerosols were reliable. Moreover, it is neces-

sary to state that the classifications of smoke aerosols pre-

sented here are subject to large uncertainty. There are un-

certainties associated with incorrect aerosol type classifica-

tion, and these uncertainties further affect aerosol extinction.

Aerosol extinction coefficients are not analyzed due to val-

ues that are too low, with high uncertainties over the TP. In

this study, four common seasons are defined, respectively, as

March to May (spring), June to August (summer), Septem-

ber to November (autumn) and December to February the

following year (winter).

The meridional circulations are examined using ERA-

Interim monthly mean reanalysis data, produced by the

European Centre for Medium-Range Weather Forecasts

(ECMWF). The spatial resolution of ERA-Interim reanaly-

sis data is 0.75◦× 0.75◦, and the vertical layers at 32 differ-

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12068 C. Xu et al.: The regional distribution characteristics of AOD

Figure 2. Monthly variations of AOD over the Tibetan Plateau. White shading indicates insufficient available data.

ent pressure levels are used (i.e., 1000, 975, 950, 925, 900,

875, 850, 825, 800, 775, 750, 700, 650, 600, 550, 500, 450,

400, 350, 300, 250, 225, 200, 175, 150, 125, 100, 70, 50, 30,

20, 10 hPa). Data are analyzed from March 2000 to Febru-

ary 2014.

3 Results and analysis

3.1 Aerosol distribution characteristics

Monthly variations of AOD over the Tibetan Plateau are

shown in Fig. 2. Seasonal variations of AOD are significant.

The monthly average of AOD for the ∼ 15-year study pe-

riod was less than ∼ 0.50 over the whole TP in spring and

summer, but less than 0.25 in autumn and winter. The high-

est AOD is shown over the Qaidam Basin on the TP in each

month. Frequent dust storms mainly lead to the high AOD

(Zhang et al., 2003; Wang et al., 2004). Human activities,

including those such as fossil fuel combustion and indus-

trial emissions over the Qaidam Basin, also contribute to the

increasing aerosol concentrations to some extent (Streets et

al., 2003; Zhang et al., 2009; Liu et al., 2015). The aerosol

load increases gradually from March to May over the north-

ern part of the TP, while it decreases from June to August.

The areas with higher aerosol loads (> 0.25) expand gradu-

ally in April and reach their maximum extent in May, which

indicates that AOD is highest in May. AOD is higher to the

north of 33–34◦ N than to the south for the whole year. The

east–west-trending mountains on the TP seem to act as a ma-

jor natural barrier for the transport of atmospheric aerosols

from north to south. The aerosol load increases slightly to the

south of 30◦ N in summer. The aerosol load over the south-

ern TP may be associated with the Indo-Gangetic Plain. A

possible explanation of this phenomenon may be the peaks

in anthropogenic emissions in the Indo-Gangetic Plain dur-

ing summer coupled with suitable atmospheric circulation.

Alpine valleys along the Himalayas (e.g., the Pulan Valley in

the western Himalayas and the Yadong Valley in the middle

Himalayas) may act as channels along which aerosols can be

transported into the southern part of the TP during the mon-

soon season. In autumn and winter, AOD over the whole TP

is mostly lower than 0.20, except in the Qaidam Basin. AOD

even decreases below 0.10 over most regions of the TP from

November to January. Unfortunately, AOD over the south-

east TP cannot always be determined in each season due to

thick cloud.

In summary, AOD is usually higher over the Qaidam Basin

than over the other parts of the TP, throughout the year. Ob-

vious seasonal variations of AOD are observed over the TP.

Dust or anthropogenic emissions may pass through the north-

ern edge of the TP, especially the Qaidam Basin, to intrude

into the TP.

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C. Xu et al.: The regional distribution characteristics of AOD 12069

Figure 3. Zonal average of AOD over the Tibetan Plateau in each

month. White shading indicates insufficient available data.

3.2 Zonal variations in aerosol properties over the TP

Although the aerosol load is quite low over the TP, the zonal

distribution pattern can be seen clearly. Figure 3 shows the

zonal monthly means of AOD for a period of over 15 years.

Monthly variations in the northern TP are different from

those in the southern TP. The aerosol load is relatively high

over the northern TP during April to June, while it is high

over the southern TP during June to August. Single monthly

peak occurs over both the northern and southern TP. The

monthly peak is observed in May and July, respectively.

Moreover, the monthly zonal peak of AOD in the northern TP

is about 1.5 times that in the southern part. Previous studies

showed similar results based on surface observations (Gobbi

et al., 2010; Xu et al., 2014). The zonal means over the whole

TP are even below 0.10 during November to January.

Overall, AOD shows clear zonal distributions. Different

monthly variation patterns of AOD are shown in the northern

part and southern part of TP. The maximum aerosol concen-

tration is observed during April to June to the region north

of 33–34◦ N, with maximum levels in May. Aerosols can be

clearly transported to the south of about 30◦ N over the TP,

with maximum levels in July.

3.3 Vertical distributions of aerosols

Dust, polluted dust, polluted continental aerosol and smoke

retrieved by CALIPSO are the major aerosol types, possi-

bly influencing the environment over the TP. The seasonal

vertical distributions of these aerosol types are discussed in

relation to the latitudinal transects. The regions from 80 to

100◦ E covering most of the TP are selected to represent

aerosol three-dimensional distributions.

Figure 4 shows latitudinal transects of accumulated dust

samples in each season. The features of aerosol three-

dimensional distributions can be concluded to seasonal vari-

ability and spatial differences. Dust generation and lofting is

always active over the Tarim Basin and Hexi Corridor all year

around, while dust occurs frequently over the northern Indian

peninsula only in spring. Detected dust over the TP increases

significantly in spring and summer, and dust occurs much

less frequently in autumn and winter. This phenomenon pos-

sibly indicates that dust can be transported into the TP in

spring and summer, while dust only occurs in Qaidam Basin

in autumn and winter. Much less dust is detected on-plateau

than off-plateau, which indicates that the TP acts as a large

barrier for the transport of dust. Furthermore, dust occurrence

decreases from the surface to the high altitude over the TP.

Most Tibetan airborne dust is concentrated at a height of less

than 7 km during spring and summer. Obvious differences

of detected dust exist between the southern and northern TP

in spring and summer. Dust occurs more frequently over the

northern part of TP than the southern part. The demarcation

between high dust occurrence over the northern TP and rela-

tively low dust occurrence over the southern TP is clear along

each longitudinal cross section. The demarcation runs east

to west across the TP, which appears to be in accord with

the distributions of monthly MISR AOD. After climbing the

northern edge of the TP, dust reduces substantially. The ex-

treme high mountains over the TP include the Kunlun and the

Tanggula. When dust encounters these major mountains one

by one, the detected dust aerosols passing through the moun-

tains decrease gradually along the longitudinal zones from

north to south in each season. The dust layer has the great-

est depth over all the longitudinal zones in spring, followed

by summer. In spring, dust layer extends from the surface to

11–12 km over the TP. Dust can be generated and lofted to

a similar altitude over the Tarim Basin and Hexi Corridor in

spring, whereas the dust layer exhibits a lesser thickness and

extends to 6–8 km over the northern Indian peninsula. The

detected aerosol layer even reaches up to upper troposphere

and lower stratosphere over the TP and the regions north of

the TP. Frequent dust activities and little precipitation may

be favorable for dust intrusion in stratosphere. Stratosphere–

troposphere exchange is currently a widely studied topic, and

aerosols intruding into the stratosphere will lead to a nega-

tive radiative forcing (Solomon et al., 2011). Previous stud-

ies have mainly focused on deep fast convective transport of

polluted air from the atmospheric boundary layer into the up-

per troposphere and lower stratosphere during Asian summer

monsoon season (Fu et al., 2006; Randel et al., 2010). The

non-volcanic aerosol layer near the tropopause was detected

vertically from 13 to 18 km based on CALIPSO observations

during the Asian summer monsoon, and AOD here has in-

creased 3 times since the late 1990s (Vernier et al., 2015).

However, our results suggest the TP and the regions north

of the TP may also act as alternative pathways for aerosols

from the troposphere to the stratosphere during the spring pe-

riod. The mechanisms of spring dust transport from the atmo-

spheric boundary layer into the upper troposphere and lower

stratosphere need further rigorous studies and discussions.

The demarcation of dust occurrence between the northern

and southern TP can be seen clearly from the surface to a

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12070 C. Xu et al.: The regional distribution characteristics of AOD

Figure 4. The detected accumulated dust samples for four seasons over the Tibetan Plateau and surrounding areas for four longitudinal

transects (80–85, 85–90, 90–95 and 95–100◦ E). Black areas represent mountain profiles along the transects. MAM denotes March to May,

JJA denotes June to August, SON denotes September to November and DJF denotes December to February the following year (the following

season divisions are the same).

height of about 6–8 km during spring, while it becomes un-

apparent at high altitudes. The dust layer can only extend to

the altitudes of 8–10 km over the TP during summer, and the

demarcation appears to be obvious at an altitude of less than

7 km. This phenomenon may indicate that dust seems to be

transported more easily through the northern edge of TP than

the southern edge during spring and summer. The dust layer

is usually at altitudes of less than 8 km over the TP in au-

tumn and winter. Due to low dust occurrence, the differences

of detected dust between the northern and southern TP are

not obvious in autumn and winter.

Figure 5 shows latitudinal transects of accumulated pol-

luted dust samples in each season. Polluted dust also affects

the environment of the TP. Nevertheless, the effect of pol-

luted dust is not as significant as dust over the TP. High oc-

currence of polluted dust is observed over the northern Indian

peninsula, except in summer, possibly due to large precipita-

tion. The occurrence of polluted dust is higher than dust over

the northern Indian peninsula except in spring. The polluted

dust is confined to the lower 5 km of the atmosphere over

the northern Indian peninsula in spring and summer, while

polluted dust is concentrated at a height of less than 4 km in

autumn and winter. The maximum height seems to be crucial

to the long-range transport of polluted dust, considering that

it is comparable to or lower than the altitude of the TP. Pol-

luted dust cannot be transported onto the TP, when its max-

imum height is lower than the southern edge. Polluted dust

also occurs to the north of the TP, but the effect is not obvi-

ous. Polluted dust decreases dramatically on-plateau relative

to off-plateau regions. Only a small amount of polluted dust

can be detected over the TP, and the number of polluted dust

samples is a bit larger in spring and summer than in autumn

and winter. The polluted dust layer also exhibits a relatively

greater thickness over the TP in spring and summer. Differ-

ences of occurrence between the northern and southern part

of TP also seem to be unapparent. Polluted dust rarely occur

in autumn and winter, especially along the longitudinal cross

section of 80–85, 85–90 and 90–95◦ E.

Figure 6 shows latitudinal transects of accumulated pol-

luted continental samples in each season. Much less polluted

continental samples are detected relative to dust or polluted

dust over the study regions. This result does not mean that

there is less urban air pollution in essence. Urban pollution

mixed with dust is classified as polluted dust. Polluted con-

tinental aerosols seem to happen more frequently in autumn

and winter over the northern Indian peninsula. Polluted con-

tinental aerosols hardly have any impact on the TP in each

season.

Figure 7 shows latitudinal transects of accumulated smoke

samples in each season. Smoke consisting of soot and OC

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C. Xu et al.: The regional distribution characteristics of AOD 12071

Figure 5. The detected accumulated polluted dust samples for four seasons over the Tibetan Plateau and surrounding areas for four longitu-

dinal transects (80–85, 85–90, 90–95 and 95–100◦ E). Black areas represent mountain profiles along the transects.

Figure 6. The detected accumulated polluted continental samples for four seasons over the Tibetan Plateau and surrounding areas for four

longitudinal transects (80–85, 85–90, 90–95 and 95–100◦ E). Black areas represent mountain profiles along the transects.

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12072 C. Xu et al.: The regional distribution characteristics of AOD

Figure 7. The detected accumulated smoke samples for four seasons over the Tibetan Plateau and surrounding areas for four longitudinal

transects (80–85, 85–90, 90–95 and 95–100◦ E). Black areas represent mountain profiles along the transects.

can be transported to the main body of the TP. More smoke

samples are detected over the Indo-Gangetic Plain rather than

the areas north of the TP. The altitude of smoke aerosol

layer is higher in summer than other seasons over the Indo-

Gangetic Basin. Although the heavy summer rains remove

a large amount of soluble gases and aerosols, less-soluble

species can be lifted to the upper troposphere in deep con-

vective clouds and then be transported away from the Indo-

Gangetic Plain by strong upper tropospheric winds. In spring,

few smoke samples are detected above the TP, and do not

show a continuous vertical distribution. This indicates that

smoke released during big fires can be occasionally uplifted

above the atmospheric boundary layer and further trans-

ported to the TP. Detected smoke samples increase lightly

over the TP during summer, and they are also not continuous

in column. Although smoke does not occur during summer

as frequently as autumn or winter over the northern Indian

peninsula, strong southerly winds blow towards the TP only

during summer. Smoke can even suspend at an altitude of

12 km over the TP in summer. Detected smoke samples are

a bit higher in the central TP in summer, which may be due

to local emission. Detected smoke samples decrease again in

autumn, and smoke usually occurs less than 7 km over the TP.

Smoke occurrence is a bit higher in the southern part of the

TP than the northern part in summer and autumn. Smoke ap-

pears to be more likely transported from the northern Indian

peninsula to the TP in summer and autumn, because higher

occurrence of smoke is shown in the adjacent regions of the

southern edge. No smoke samples are detected over the TP

in winter.

In summary, pure dust is found to be the main aerosol type

above the TP, and dust mixed with pollution or smoke occa-

sionally occurs. Smoke only has a little effect on the TP in

summer, while urban pollution does not contaminate the en-

vironment of the TP individually. Much fewer aerosols are

detected on-plateau than off-plateau, which indicates that the

TP acts as a natural barrier. Dust and polluted dust exhibit

more thickness in spring and summer above the TP. The

bulk of dust is concentrated at a height of less than 7 km

during spring and summer. Different dust occurrences be-

tween the northern and southern TP can be found clearly

in spring and summer. No significant differences are found

in the occurrences of polluted dust, smoke or polluted con-

tinental aerosols over the northern and southern TP. Smoke

aerosols may be more likely to come from the northern In-

dian peninsula in summer.

3.4 Possible factors contributing to the aerosol

distribution pattern

The TP has quite pristine atmospheric conditions; that is

to say, few aerosols come from local contributions. Zhang

et al. (2015) showed that local emissions contributed only

a small percentage of BC in the Himalayas and Tibetan

Plateau. There are no obvious sources of biomass burning

on the TP (Mouillot and Field, 2005). As the most prominent

aerosol type on the TP, airborne dust on the TP mostly comes

Atmos. Chem. Phys., 15, 12065–12078, 2015 www.atmos-chem-phys.net/15/12065/2015/

C. Xu et al.: The regional distribution characteristics of AOD 12073

Figure 8. The meridional circulation at 95◦ E in the four seasons. The black shading shows the altitudes of the Tibetan Plateau at 95◦ E.

from the surrounding regions. A negative or significant neg-

ative relationship between AOD and wind speed was found

over most regions of the TP based on previous results (Ge

et al., 2014). Therefore, long-range transport of aerosols pri-

marily impacts the atmospheric environment over the TP, and

this study is not focused on the contribution of inner sources

on the TP. The aerosol load has significant seasonal varia-

tions. Interestingly, the pattern of seasonal variations over the

northern TP is different to that of the southern part; this could

be due to many factors, including the emission sources, high-

altitude terrain and atmospheric circulation.

Much higher aerosol loads are observed over the surround-

ing regions of the TP. AOD peaks during spring and sum-

mer over Tarim Basin. Strong anticyclonic wind anomaly

at 500 hPa and enhanced easterly wind at 850 hPa over the

Tarim Basin during spring and summer are good for dust

entrainment, vertical lofting, and horizontal transport (Ge et

al., 2014). The Indo-Gangetic Basin, encompassing most of

northern India peninsula, extends from Pakistan in the west

to Bangladesh in the east. The Indo-Gangetic Basin is one of

the most heavily populated regions of the world. There are

a large quantity of emissions of biomass burning and fos-

sil fuel over south Asia, adjacent to the TP (Ramanathan

et al., 2005). AOD over the Indo-Gangetic Basin can reach

extremely high values throughout the year, peaking during

spring and summer due to enhanced emission of natural

aerosols (Dey and Di Girolamo, 2010). Furthermore, aerosol

layers exist above the TP over the northern Indian peninsula

and Tarim Basin during spring and summer. Dust and pol-

luted dust layers exhibit a relatively greater thickness over the

regions north of the TP than the regions south of the TP dur-

ing spring and summer. The aerosol concentrations and the

heights of aerosol layers over the surrounding regions have a

great influence on the transport of aerosols.

The high terrain acts as a natural barrier for the transport of

atmospheric aerosols from the surrounding polluted regions

to the main body of the TP due to its topographic characteris-

tics. Therefore, AOD over the TP is much lower than that of

surrounding regions, such as the Taklamakan Desert and the

Indo-Gangetic Plain. The aerosol distributions are impacted

by the mountain ranges on the TP. The high aerosol load oc-

curring over the northern part seems to be associated with

lower altitudes, and a relatively low aerosol load along the

southern edge seems to be associated with the higher alti-

tudes. The aerosol layer firstly needs to be present higher

than the TP elevation, which is a requirement for aerosol

transport to the TP. The southern edge is much higher than

the northern edge of the TP. The transport of aerosols from

north of the TP seems easier than from south of the TP.

Aerosols may only pass through alpine valleys along the Hi-

malayas to intrude into the TP, while a broader northeastern

edge – especially the Qaidam Basin – seems to provide trans-

mission channels. When aerosols pass across the northern

edge, the major natural obstacles they encounter are several

www.atmos-chem-phys.net/15/12065/2015/ Atmos. Chem. Phys., 15, 12065–12078, 2015

12074 C. Xu et al.: The regional distribution characteristics of AOD

Figure 9. Spatial distribution of wind at 500 hPa over the Tibetan Plateau in the four seasons.

mountains. The Kunlun and the Tanggula mountains act as

barriers which block aerosols. It is then difficult for aerosols

to spread further southward. The Gangdise and the Nyain-

qêntanglha mountains are located around 30–31◦ N on the

southern TP. These mountains possibly act as a natural bar-

rier for aerosols passing across the southern edge, and curb

the spread of pollution further northward to the main body of

the TP. The demarcation extending to 6–8 km seems to exist

only in dust aerosols, while it is not apparent in other types of

aerosols due to their small amounts. The high-altitude terrain

located around 33–35◦ N in the middle of the plateau appears

to be geographically identical to this natural demarcation.

Not only does the whole TP block the atmospheric aerosols,

but also the extreme high mountains on the TP cause an ob-

struction to the transport of aerosols.

Atmospheric circulation also greatly impacts the seasonal

aerosol variations. Figure 8 shows the annual average ver-

tical wind fields, and Fig. 9 shows the wind vector field at

500 hPa. A longitude of 95◦ E, corresponding to the Qaidam

Basin, is chosen to analyze the vertical atmospheric circu-

lations. During the spring period, the northern air flows and

southern air flows intersect at about 31–32◦ N over the TP.

Aerosols above the TP are mostly from the northern side

of the TP in spring, and aerosols originating from the Indo-

Gangetic Basin may only affect the area south of 31–32◦ N

on the TP. Northwesterly and westerly winds at 500 hPa pre-

vail over the Indo-Gangetic Plain during spring. Although

high aerosol load occurs and aerosol layer exists at 5 km

during spring, this atmospheric circulation does not bene-

fit to the transport of air pollutants from the northern India

to the TP. Kuhlmann and Quaas (2010) reported that around

40 % of elevated desert dust or polluted dust from Iran and

Pakistan can be advected towards the southern slope of TP.

During summer, the airflows originating from both the north-

ern and southern sides of the TP carry aerosols to the main

body of the TP. The Indo-Gangetic Basin is dominated by

a cyclonic circulation system at 500 hPa. In addition, ver-

tical circulation indicates strong updrafts below 200 hPa to

the south of the TP. Updrafts are also shown on the whole

TP. With the summer monsoon developing, the southwest-

erly winds at 500 hPa reach the northward maximum extent

over the TP. Furthermore, the northern atmospheric circula-

tion system and south Asian monsoonal system meet around

34–35◦ N in the middle of the TP. The atmospheric circula-

tion promotes the transport of aerosols from the Tarim Basin,

Qaidam Basin and Indo-Gangetic Basin to the main body

of the TP. It might be reasonably deduced that atmospheric

circulation possibly leads to the formation of different sea-

sonal variation patterns of aerosols between the northern and

southern TP. Dust particles coated by pollution acids can

provide the predominant source of cloud condensation nu-

clei (Ma et al., 2010). Urban emissions consisting of hygro-

scopic compounds could be deposited by heavy precipitation

and reduce substantially over the northern Indian peninsula.

Atmos. Chem. Phys., 15, 12065–12078, 2015 www.atmos-chem-phys.net/15/12065/2015/

C. Xu et al.: The regional distribution characteristics of AOD 12075

Some less-soluble species can be lifted to the upper tropo-

sphere in deep convective clouds and then be transported to

the TP by strong upper tropospheric winds. The strong south-

westerly winds can provide the dynamical conditions for the

transport of aerosols during monsoon season, which leads to

the highest aerosol load occurring in July over the southern

TP. The aerosol particles continue to reduce drastically after

passing through the Himalayas Mountains due to the physical

blocking. Consequently, the aerosol loading and occurrences

are still higher in the northern TP than the southern TP dur-

ing summer. Seasonal variations of atmospheric circulations

lead to the relatively higher aerosol loads in spring and sum-

mer on the TP. During autumn, southwesterly winds mainly

prevail south of 31–32◦ N at 500 hPa on the TP with the re-

treat of monsoon. During the winter period, wind fields are

similar to autumn but with higher wind velocities. Westerly

winds at 500 hPa gradually prevail over the Indo-Gangetic

Basin from autumn to winter. Moreover, our results reveal

that the aerosol layer usually exists at an altitude of less than

4 km over the Indo-Gangetic Basin during autumn and win-

ter. Aerosols originating from Indo-Gangetic Basin cannot

be lifted to a height higher than the elevation of the southern

edge during autumn and winter. Conversely, some elevated

dust aerosol layers higher than the northern edge of the TP

are observed north of the TP during autumn and winter. It

is difficult for aerosols transported from the Indo-Gangetic

Basin to the TP during autumn and winter seasons. Elevated

dust aerosols from the north of the TP can be possibly trans-

ported to the TP coupled with northward winds.

4 Concluding remarks

This study identifies the patterns of aerosol variations over

the TP using the 15-year MISR data. Furthermore, the ver-

tical distributions of dust, polluted dust, polluted continental

aerosols and smoke retrieved by 8-year CALIPSO data are

also investigated over the TP. The possible reasons for the

temporal variations and spatial distributions of aerosols are

discussed.

The aerosol load exhibits obvious seasonal variations over

the investigated regions, with higher AOD observed during

spring and summer. Two different kinds of seasonal patterns

of AOD are observed over the TP. The maximum monthly

AOD over the northern TP occurs in May, while AOD over

the southern TP peaks in July. AOD shows much higher val-

ues over Qaidam Basin than other parts of the TP through-

out the year. Monthly AOD usually shows higher values over

the northern TP than the southern TP. Dust is found to be

the major aerosol type above the TP, while polluted dust and

smoke aerosols slightly affect the atmospheric environment

on the TP. Therefore, the seasonal variation and spatial pat-

tern of aerosol load are largely associated with dust occur-

rence. Both the occurrence and the thickness of airborne dust

also reach maximums over the TP in spring. The dust layer

over the TP can reach up to the upper troposphere and lower

stratosphere in spring (altitudes of ∼ 11–12 km), while the

altitude of dust layer is much lower in other seasons. Higher

dust occurrence in the northern TP and lower dust occurrence

in the southern TP are observed during spring and summer.

This dividing line is located around 33–35◦ N in the middle

of the plateau. In addition, this demarcation extends from the

surface to an altitude of 6–8 km. However, this demarcation

is not observed in the distributions of other aerosol types due

to their low occurrences.

The seasonal variations and distribution characteristics of

the aerosol load on the TP are possibly affected by many fac-

tors, including emission sources, the height of aerosol layer,

atmospheric circulation and the topography of the TP. High

concentrations of aerosols exist during spring and summer

over the surrounding regions including the Indo-Gangetic

Basin and the Tarim Basin. Furthermore, aerosol layers ex-

ist above the TP elevation during these two seasons. These

conditions are favorable for aerosol transport from the sur-

rounding regions to the TP. Different seasonal variations over

the northern and southern TP are closely associated with at-

mospheric circulation system. Atmospheric circulations also

greatly control the potential maximum limit of aerosol trans-

port. However, the actual distribution patterns of aerosol are

not completely consistent with the maximum extent of air-

flows. The mountains on the TP may effectively block the

transport of aerosols. It is the possible reason that a dividing

line of different dust occurrences between the southern and

northern TP exists in the middle of the TP during spring and

summer.

Acknowledgements. This research was funded by the Chinese

Academy of Sciences (XDB03030201), the National Natural

Science Foundation of China (91337212, 41275010, 41375009),

the CMA Special Fund for Scientific Research in the Public Interest

(GYHY201406001) and EU-FP7 projects “CORE-CLIMAX”

(313085). This study was also funded by the CAS “Hundred

Talent” program (Weiqiang Ma). We would like to thank the editor

and two anonymous referees for their very valuable comments

greatly improving the paper. The MISR data were obtained

from the NASA Langley Research Center Atmospheric Science

Data Center. The CALIPSO data were obtained from the NASA

Langley Research Center Atmospheric Science Data Center. The

ERA-Interim data were produced by ECMWF. The first author

would like to acknowledge Changgui Lin, You He and all the other

group members for their help in preparing the paper.

Edited by: H. Su

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